1. Field of the Invention
The present invention relates generally to semiconductor devices, and more particularly to semiconductor devices having a semiconductor thin film as its active layer and the manufacturing method thereof. The invention also relates to thin film semiconductor transistors with an active layer made of crystalline silicon films.
2. Description of the Prior Art
In the recent years semiconductor thin-film transistor (TFT) devices are becoming more widely used in the manufacture of electronic parts or components, particularly reduced-thickness display devices and digital integrated circuit (IC) packages, as the speed and cost advantages of these devices increase. As such electronics require higher packing density, higher speed, and lower power dissipation, TFTs become more critical in performance and reliability. Some prior known TFTs come with a silicon thin film formed on a substrate with a dielectric surface, which film typically measures several tens to several hundreds nanometers (nm) in thickness.
Typically, the TFT has an active region as defined between spaced-apart source and drain regions for selective formation of a channel region therein. The active region, namely, the channel formation region, as well as its associated source/drain junction regions may play an important role to determine the performance of TFT as a whole. This can be said because the resistance of a current path from the source to drain through the channel, or the mobility of minority charge carriers, can strictly reflect the overall electrical characteristics of TFTs.
Conventionally, amorphous silicon films have been generally employed as the semiconductor thin film constituting the active layer of TFTs. These amorphous silicon films may be fabricated by plasma chemical vapor deposition (CVD) and low pressure thermal CVD techniques.
Unfortunately, the use of such amorphous films is encountered with a problem that where TFTs are required to exhibit higher operation speeds, amorphous films are incapable of trace such trend due to its inherently lowered mobility of charge carriers. To this end, silicon thin films with enhanced crystallinity (to be referred to as the xe2x80x9ccrystalline silicon filmxe2x80x9d hereinafter) should be required.
One prior known approach to form such crystalline silicon film on a substrate has been disclosed, for example, in Published Unexamined Japanese Patent Application (PUJPA) No. 6-232059 to be assigned to the present assignee. In this prior art a chosen metallic element is employed to facilitate or accelerate crystal growth of silicon, which is subject to thermal or heat treatment at a temperature of 550xc2x0 C. for four hours. With this, resultant crystalline silicon film offers enhanced crystallinity. A similar approach has also been disclosed in PUJPA No. 6-244103.
Another prior art approach has been disclosed in PUJPA No. 7-321339, wherein a similar technique is used causing silicon to grow in substantially parallel to the crystal plane of a carrier body, such as a supporting base plate, i.e., substrate. The resulting crystallized region is called the xe2x80x9clateral growth regionxe2x80x9d in some cases.
The lateral growth region thus formed using the above technique is improved in crystallinity due to the fact that columnar or capillary crystals are gathered with the crystal growth directions being well aligned to one another. The use of such region to form an active layer or layers may contribute to an increase in performance of TFTs.
As the semiconductor manufacturers are commercially demanded to further improve the TFT speed endlessly, even the TFTs with such lateral-growth films as the active layer thereof will be unable to catch up the strict demands due to their inherent limitations as to improvements of the crystallinity.
Advanced active-matrix liquid crystal display (LCD) devices or passive LCDs which employ thin-film transistors (TFTs) for respective picture elements or xe2x80x9cpixelsxe2x80x9d are examples. The LCDs of these types incorporate peripheral circuitry which includes driver circuits for electrically driving an associative LCD pixel array, image data processor/controllers for handling video signals in a desired format, a memory array for storage of several kinds of information items, and the like. Of those circuit components, the data processor/controllers and memory array are strictly required to be equivalent in performance to presently available advanced integrated circuit (IC) chips as fabricated using single-crystalline wafers. Accordingly, where these LCD driver circuits are integrated on a substrate by use of a semiconductor thin film as formed on the substrate surface, it is required that such thin film exhibit the maximum similarity in nature to the crystallinity of single crystals. Unfortunately, none of the prior art proposed are capable of overcoming this problem. One reason for this is that the lateral growth silicon films do not come without accompanying a problem that reliability and productivity remain lowered due to the fact that the metallic element as used for acceleration of crystal growth might continue to reside within resultant silicon films, which disadvantageously serves to degrade the reproducibility. This is a serious bar to a further advance in semiconductor fabrication technology.
It is therefore an object of the present invention to provide a new and improved approach that avoids the problems faced with the prior art.
It is another object of the invention to provide a new and improved semiconductor device capable of avoiding the problems faced with the prior art as well as the method for forming the same.
It is a still another object of the invention to provide a semiconductor integrated circuit device capable of offering enhanced performance and reliability without having to make use of single-crystalline semiconductor wafers.
It is yet another object of the invention to form a mono-domain region having superior crystallinity equivalent to en single-crystalline on a carrier body with a dielectric surface.
It is a further object of the invention to provide a semiconductor device having an active layer overlying a substrate with a dielectric surface and being made of a mono-domain region that is equivalent in crystallinity to single-crystalline materials.
To attain the foregoing objects, in accordance with one aspect of the present invention, a specific device is provided which has a carrier body with a semiconductor thin film being formed on an insulating surface of the carrier body, featured in that the thin film includes a mono-domain region including a mixture of a plurality of crystals substantially parallel to the carrier body, wherein the crystals may be columnar crystals and/or capillary crystals.
In accordance with another aspect of the instant invention, there is provided a semiconductor thin film on a dielectric surface of a carrier body. The thin film includes a mono-domain region containing a mixture of a plurality of crystals substantially parallel to the carrier body. The crystals may be columnar crystals and/or capillary crystals. Very importantly, the mono-domain region does not include any crystal grain boundary therein. Part of the thin film constituting the mono-domain region contains hydrogen and halogen elements at a carefully chosen rate that is equal to or less than five (5) atomic percent. Preferably, the halogen may be chlorine, bromine and/or fluorine.
In accordance with still another aspect of the invention, the semiconductor device makes use of the mono-domain region exclusively for formation of the active layer thereof. In this case, no grain boundaries are present within the mono-domain region.
In accordance with yet another aspect of the invention, a method of forming a semiconductor thin film is provided, which method including the steps of forming by low pressure chemical deposition an amorphous silicon film on a carrier body having a dielectric surface, selectively forming a silicon oxide film on the amorphous silicon film, retaining a metallic element for facilitation of crystallization of the amorphous silicon film, altering by a first heat treatment at least part of the amorphous silicon film to a crystalline silicon film, removing the silicon oxide film, performing a second heat treatment in a chosen atmosphere containing halogen elements to form a thermal oxide film containing therein halogen on the amorphous silicon film and/or the crystalline silicon film while allowing the crystalline silicon film to change in nature to a corresponding mono-domain region, and removing the thermal oxide film. The resultant mono-domain region is then employed for formation of an active layer of the semiconductor device.
It should be noted here that the term mono-domain region, is used herein to refer to lateral growth crystal region as formed using the semiconductor thin film manufacturing method of the invention, by taking account of the fact that this region has superior crystallinity enhanced sufficiently to be regarded as the single crystal materials in substance. A principal feature of the mono-domain region is that no grain boundaries are found within its entire region, and accordingly any lattice defects or dislocations are suppressed or eliminated which are otherwise occurred due to presence of transitions and stacking fault (interlayer defects). Another feature is that the mono-domain region avoids inclusion of any metallic elements otherwise acting to badly influence the fundamental characteristics of the semiconductor device.
It should also be noted that the absence of crystal grain boundaries also covers in meaning the fact that even if a few grain boundaries are present, these remain electrically inactive. As such electrical inactive grain boundaries, there have been reported the {111} twin-crystal grain boundary, {111} stacking fault, {221} twin-crystal grain boundary, and {221} twist twin-crystal grain boundary (R. Simokawa and Y. Hayashi, Jpn. J. Appl. Phys., 27 (1988) at pp. 751 to 758).
The present inventors consider that crystal grain boundaries contained in the mono-domain region remain as electrically inactive grain boundaries at increased possibility. In other words, even where some boundaries might be observed therein, such are electrically inactive regions which will no longer affect the movement of charge carriers therein: In this sense, these boundaries if any remain electrically xe2x80x9ctransparentxe2x80x9d to the flow of internal current.
These and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.